1. Field of the Invention
The present invention relates to a system for a substance separation and energy recovery by thermal treatment; and more particularly, to an energy recovery system used to process low calorific value substances.
2. Description of the Related Art
Most substances with high moisture content have low calorific value. During combustion, these low calorific value substances results in low combustion temperature and air pollutants due to incomplete combustion.
Therefore, most high moisture content substances usually will be dried to remove water and increase heating value (as in the cases of coal drying or sludge drying) prior to combustion. A substance with lower moisture content helps to achieve a higher combustion temperature and better combustion stability.
The most popular drying method is hot air drying. Air is heated to a relative high temperature and is introduced to contact with the substances to be dried. Thus remove the water in the substance through evaporation.
The conventional utilization of the high moisture content and low calorific value substance as fuel has the following disadvantages: first of all, energy recovery of high moisture content substances through a direct combustion process results in incomplete combustion and air pollutants. Water in substance absorbs large amount of latent heat and evaporates to steam during combustion process. Combustion flue gas contains lots of the incondensable gases such as oxygen, nitrogen and carbon dioxide. The existing of the incondensable gases in flue gas will lower the dew point (condensation temperature of steam) of the flue gas. Therefore, the latent heat of the steam is very difficult to recover.
Most of the high moisture content substances prefer to remove their water in the substances through hot air drying prior to combustion. However, the hot air (heating media) containing incondensable gases results in low dew point of the air-steam mixture. Therefore, the latent heat absorbed by water during drying is also hard to recover.
Another disadvantage of conventional hot air drying is the substance to be dried will be oxidized and even catch fire due to the existing of oxygen. Thus, fire prevention equipments are necessary or lower the hot air temperature to reduce the chance of catching fire.
Furthermore, conventional hot air drying will produce large amount of pollutants such as dioxins and odors. Post treatments of the exhaust gases are required before discharging to ambient.